Recently considerable work has been directed to the use of the rugged induction machine as a generator, with its excitation supplied not by conventional means but a switching system resembling a conventional inverter circuit. Such an arrangement is described and claimed in U.S. Pat. No. 3,829,758-Studtmann, entitled "AC-DC Generating System," which issued Aug. 13, 1974, and is assigned to the assignee of this invention. FIG. 6 of that patent shows an arrangement in which a switching system, resembling an inverter circuit, supplies the excitation for an induction machine, without any necessity for a capacitor bank, additional machine or other external excitation arrangement. That arrangement provided both a d-c output voltage on the normal inverter bus conductors, and an a-c output voltage from the induction machine connections. As noted in that patent, the frequency of the generator voltage was essentially proportional to generator speed, deviating only by an amount equal to the slip frequency. To obtain a constant or controlled output frequency independent of shaft speed, generally termed a variable speed, constant frequency (VSCF) system, the inverter switching frequency can be modulated as disclosed and claimed in the application entitled "Modulated Induction Generator," Ser. No. 568,746, filed Apr. 16, 1975, which is assigned to the assignee of this invention. That system provides an a-c output voltage at a frequency which is independent of shaft speed, but this frequency must be substantially lower than the generator operating frequency. In some systems, this can be a significant drawback as the required generator frequency can become very high, leading to possible inefficient operation. In addition, that system results in modulation of the entire magnetic field of the machine, so that production of multiphase outputs requires the use of isolated magnetic circuits. For example, to achieve a three phase output, three isolated magnetic circuits or three machines would be required. This may result in a size and weight penalty. Thus the purpose of the present invention is to obviate the two major problems just described.
It is therefore a primary object of this invention to provide a generating system, the output voltage of which alternates at a frequency independent of the generator frequency, and which in fact may be above or below the generator frequency.
A further object of this invention is to provide a system in which the magnetic field of the machine remains built up, so that multiphase outputs can be obtained from one conventional three-phase machine.
A more specific object of this invention is to provide a logic circuit for effecting the precise operation of the switching system to produce the a-c output voltage for use with a-c loads.